Cleaning stone

I want to comment on the recent queries and responses about stone
cleaning. Let's back up and look at some basic issues, for the
benefit of those of us who have been involved for many years and for
others who are considering a first stone treatment.  At the risk of
being obvious, I want to point out that it is not always necessary
or appropriate to clean stone.  This applies to stone sculpture
indoors as well as to monuments and buildings.

Cleaning stone is an irreversible process.  The goal of the
treatment should be clearly understood by the owner/agent/curator
and conservator. In order to prevent physical damage or loss of
information during cleaning, all the examination and testing
procedures familiar to professional conservators should be carried
out.  The stone object will be best served when technical decisions
about materials and techniques evolve from collaborative
deliberation and review of testing results.

There have been several replies on this list recommending JOS
cleaning.  JOS is a low pressure, micro abrasive system that was
developed commercially in Germany.  It is used for some other
materials besides stone but this note addresses only use on stone.
There are several types of JOS equipment and a selection of nozzles
presently available. The JOS cleaning system accommodates a variety
of media.

Most common in the U.S. at this time is the use of a nozzle that can
deliver air, media and water simultaneously.  The most frequently
specified media, I believe, is glass.  Anyone can purchase the
equipment and order the media but operators of the system are
supposed to be trained by a representative of the parent company.
Some conservators own and operate JOS systems, but stone specialist
contractors are carrying out most of the work currently in progress.

The JOS micro-abrasive system, maintained at low pressure, can
provide the best solution for cleaning stone in some outdoor
situations but by no means all situations.  Not only has the JOS
system been observed to have different cleaning effects on different
stone, it has been observed to have dissimilar results on the same
stone at different locations.

Testing is essential. Competency and conscientiousness on the part
of the operator is fundamental.  It would be misleading to say that
JOS does not damage stone without defining "damage". There is
definitely some material removed from the surface, although
extremely small.  This is documented in a publication:

    Mossotti, V.G., Eldeeb, A. R., Fries, T.L., Coombs, Mary Jane,
    Naude, V.N., Soderberg, Lisa and Wheeler, G.S., in press The
    effect of selected cleaning techniques on Berkshire Lee marble:
    a scientific study at Philadelphia City Hall: U.S. Geological
    Survey Professional Paper 1635 (CD-ROM).

It will be available in early 2002 from

    USGS Information Services
    Box 25286
    Denver Federal Center
    Denver, CO 80225
    888-275-8747

The publication reports on a collaborative study in 1995 by
conservators, architects and scientists. It is based on field tests
of 8 methods in 4 different soiling conditions.  Over 150 cores were
taken for analysis by USGS scientists.  Results of analysis were
reviewed by the team.  JOS, using dolomite as the media, was one of
three treatments that fulfilled the goals of the client and the
criteria of the design team. The JOS system, combined with water
misting to reduce sulfation crusts, is being used currently on the
marble ashlar and carvings at Philadelphia City Hall. Supervision of
the contractor's JOS-trained operators by the architect and
conservator is very important to maintain awareness of cleaning
goals and treatment design.


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                  Conservation DistList Instance 15:26
               Distributed: Wednesday, September 26, 2001
                       Message Id: cdl-15-26-002
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